1. Field of the Invention
The present invention relates to a circuit board for mounting an optical device to be used in light/electricity conversion or electricity/light conversion, more particularly to an optical device circuit board to be used for an optical reader, optical image forming device, optical printer or the like.
2. Description of the Related Art
Recently, an electronic circuit has been applied in a wide variety of fields from, for example, business equipment to household articles or toys, and accordingly the development of small type, light weight, high speed and accurate equipment is now in progress. Also in the field of various kinds of image input terminal equipment including a high-speed facsimile, a scanner, a white board copying machine, or such as a copying machine represented by electrophotography, there is an increasing demand for the equipment of high quality, high resolution, capable of handling a half-tone image while having a simple, compact structure and also can be produced at low cost.
In the image input terminal equipment as above, optical information is converted to an electric signal by means of an image sensor. FIG. 1 is a typical cross sectional view illustrating an example of the structure of a conventional image sensor. In FIG. 1, a shading layer 52 is provided on a glass substrate 51 with a slender part cut out to form a slit 52a. The slit 52a extends in the direction perpendicular to the surface of the paper showing the figure, and FIG. 1 is a cross sectional view taken along the plane perpendicular to the longitudinal direction of the slit 52a. A transparent insulation layer 53 is provided in a manner to cover the shading layer 52 and slit 52a, and a plurality of light receiving devices 56 and electrodes 54 electrically connected therewith respectively are provided on the transparent insulation layer 53. The light receiving devices 56 are disposed along the longitudinal direction of the slit 52a. A transparent protective layer 55 is provided so as to cover the whole area of the transparent insulation layer 53, electrode 54 and light receiving device 56, and on the surface of the transparent protective layer 55, i.e., on the outermost surface of the image sensor, a transparent conductive layer 57 is formed. This image sensor is manufactured as a unit.
For reading a letter or a figure on a manuscript 59 by using this image sensor, it is enough when the manuscript 59 is placed on the surface of a transparent conductive layer 57 so that the manuscript can be moved in the direction of an arrow shown in the figure with a dotted line by means of a roller 60, and further provide a light source 58 on the side of the glass substrate 51 to make light 61 from the light source 58 pass through the slit 52a and reflect on the surface of the manuscript 59 to enter into the light receiving device 56. By transferring the manuscript in this state by means of the roller 60, an image is inputted by line scanning.
Since it has been necessary to provide a number of light receiving devices 56 along the longitudinal direction of the slit 52a, a light detecting device with an amorphous semiconductor has been used as the light receiving device 56. However, the amorphous semiconductor device has defects such as low photo detecting sensitivity, the characteristic tends to be greatly deteriorated when exposed to light for a long time period and a small responding rate. Therefore, it is difficult to operate at a rapid reading rate when the amorphous semiconductor is employed as the light receiving device.
On the other hand, a light receiving device using a crystal semiconductor is characterized by the excellent photo sensitivity and high responding rate. Therefore, a scanner composed of the light receiving device based on the crystal semiconductor allows high speed reading. When the image sensor is made by using a crystal base semiconductor device, it is difficult to make it as a unit, and hence the crystal based semiconductor device (LSI chip) and circuit board are produced separately and then a plurality of the light receiving LSI chips are combined with a circuit board in a multichip method. In this way, a highly sensitive, a high speed reading type image sensor is manufactured. In this case, each LSI chip can be checked whether it is good or bad before it is assembled into a sensor device. Therefore, by selecting only good light receiving LSI chips, a circuit board for optical LSI chips can be checked before assembling, it becomes possible to increase the yield of the sensor thereby achieving the cost down thereof.
The present inventor et al. started study for further improving the image sensor. The present inventor et al. first studied the structure which allows a minimum clearance between LSI chips and a manuscript in order to realize a low output light source which is to be used for reading the manuscript. More particularly, as a transparent circuit board for mounting the LSI chips, the inventor et al. studied an extremely thin glass substrate of 0.1 mm or less in thickness, or studied to use a plastic film as a substrate. The LSI chip described here is of a bare chip type or has a pad made of gold or aluminum provided thereon as an external connection terminal.
Now, a circuit board having the base of a plastic film or a sheet will be described. Plastic is flexible, and an electrode can be formed easily with high productivity on the circuit board made of plastic by means of a spattering or a vacuum evaporation method while keeping the board in a low temperature. At present, circuit boards with the base made of a plastic film or a sheet are widely used in the part of the equipment subject to an extremely severe bend or needed to reduce its weight, but the use of circuit boards of this kind is expected to increase in a more wide area accompanying the progress of higher integration and further reduction in size and weight of the electronic equipment.
However, the present inventor et al. have found that since a circuit board composed of plastics is made of an insulation and tends to be charged, static electricity generated by friction is liable to accumulate on the surface thereby influencing the semiconductor device mounted on the circuit board. Practically, an error or disorder is generated in the output signal resultantly causing a noise or inducing an error motion in the circuit connected to this circuit board. When it is used in a contact type image sensor, and a manuscript and this circuit board rub together, the charge gives a serious influence. There is a stray or parasitic capacitance when the insulating layer is between two conductive layers. When optical semiconductor LSI chips are in the insulating layer, the capacitance will cause noise or crosstalk to the semiconductor LSI chips. The stray capacitance becomes great in the case of a longitudinal image sensor.
In the optical reader, light from a light source is irradiated onto a manuscript which is moving under a film-like transparent circuit board for reading the manuscript, while the light from the light source reflects in multipath between the manuscript and the circuit board. The present inventor et al. have also detected that inasmuch as the light receiving device for reading receives the multipath-reflected light thereby causing an error in the output signal of the light receiving device, sometimes reading is not correctly performed.
The argument similar to the above is also applicable to an optical image forming LSI chip which has a number of light emitting devices distributed for forming an image on a photosensitive matter by using light from these light emitting devices.
Further, with reference to a multichip type optical device circuit board having a plurality of reading LSI chips (light receiving LSI chips) or image forming LSI chips (light emitting LSI chips) mounted thereon, it is necessary to provide on the optical circuit board a number of electrodes to be used for one to one connection with these devices. Further, it is necessary to provide on or outside of the circuit board an input and output circuit for electrically activating these devices and receiving signals therefrom. Therefore, in this case, a skill is required for electrically connecting the input/output circuit and the optical device circuit board, and further the connection reliability is liable to be lowered. If the reading area is enlarged by using such as an image sensor, light receiving devices are needed additionally on the circuit board corresponding to the increased reading area resulting in the remarkable increase of electrodes on the board.
Therefore, in order to connect the light receiving LSI chips and the light emitting LSI chips in a manner of matrix wiring or the like, an aggregated circuit board is provided between the optical device circuit board and the input/output circuit. In this case, the connection may be performed such that the electrical connection between the optical device circuit board and the aggregated circuit board is first performed and then the aggregated electrodes on the aggregated circuit board and the input/output circuit are connected respectively. By providing the aggregated circuit board, the input/output circuit is allowed to access to aggregated electrodes of the number less than the number of the optical devices, and hence it becomes possible to activate an optional device or receive a signal from an optional device. However, the aggregated circuit board requires an additional cost for preparing thereof, and manpower is further needed for electrical connection between the optical device circuit board and the aggregated circuit board. When electro-optical devices are increased in numbers, practically the number of aggregated electrodes is increased approximately proportionally to the number of increased electro-optical devices. Therefore, a method which can provide this aggregated circuit board or aggregated electrode with low cost and high reliability is requested. It is also desired to prepare an optical device circuit board together having a function of the aggregated circuit board.